Dam excavation shaft and trench shuttering

The Department's description

Exterior description and setting

Section of cast-metal shafting and shuttering set into terrace overlooking discharge channel. Exhibition feature to demonstrate construction of Silent valley dam. Comprises two circular shafts at 10.5m (34ft 6in) centres to depth of 2.3m (7ft 6in), separated by two parallel walls of segmental cast-metal plates at c. 1.8m (6ft) spacing. Each shaft comprises a series of rings, 3.8m (12ft 6in) in external diameter by 0.46m (18in) deep, internally flanged and bolted together. Each ring formed by ten cast-iron segments, also bolted to one another on internal flanges. Each segment is embossed RP&S (= ?Pearson & Son, contractor). Baulks of timber run between the segmental walls. A post and wire fence surrounds.

Historical information

The original plans drawn up by the Belfast City and District Water Commissioners for the construction of the Silent Valley dam envisaged that bedrock would be reached some 50ft below the surface. This was based on corings which, unfortunately, had hit glacial erratics. The contractors found it impossible to establish true bedrock at this level, but were prevented from continuing downwards due to water ingress. Having taken the matter to arbitration, work was continued on an exploratory basis. Various methods of dewatering were tried, and eventually these vertical shafts, which were filled with compressed air, provided the solution.

Their purpose was two fold: 1) to establish bedrock into which a cut-off trench (to prevent water seepage under the dam) was to be excavated; 2) to dewater the area around the intended embankment, so that the overburden above the bedrock could be removed and the cut-off trench excavated.

In operation, segments were added to the bottom of the previously laid ring as excavation progressed. Seepage was minimised by the use of compressed air inside the shafts (with an air lock and decompression chamber at top). A number these exploratory shafts were inserted along the proposed line of the dam between 1927 and 1929, the work being carried out by S. Pearson & Son (London). At one point bedrock was found to be at a depth of 212ft below ground level.

Once bedrock had been reached, and the groundwater pumped out, the overburden was removed from the surface downwards. Excavation of a 6-7 ft wide trench in the rock began. As this progressed, segmental castings were bolted together between the shafts along the walls of the trench; the timber baulks were used to keep them at a uniform distance apart. Bedrock was excavated to a depth of upwards of 50ft, and the shuttered trench thus formed filled with concrete between 1929 and 1930. The upper sections of the shafts were removed prior to the formation of the earthen dam above this trench (the bottom sections were used as shuttering for the concrete trench).

Secondary Sources:
1. G. McIldowie ‘The construction of the Silent Valley reservoir, Belfast water supply’ in Proc. Institution Civil Engineers, N. Ireland Branch (Belfast 1934).
2. City & District Water Commissioners, ‘The Mourne Scheme’ (Belfast 1959).

Criteria for listing: architectural interest

G. Innovatory Qualities
K. Group value

Criteria for listing: historic interest

Z. Rarity

Evaluation

Section of cast-metal shafting and shuttering set into terrace overlooking discharge channel. Exhibition feature to demonstrate construction of Silent valley dam. Comprises two circular shafts at 10.5m (34ft 6in) centres to depth of 2.3m (7ft 6in), separated by two parallel walls of segmental cast-metal plates at c. 1.8m (6ft) spacing. Each shaft comprises a series of rings, 3.8m (12ft 6in) in external diameter by 0.46m (18in) deep, internally flanged and bolted together. Strictly these items are not in situ and are incomplete. However, in view of their close spatial proximity to the dam (HB16/02/064), and historical and technical interest, it is considered that they form an integral part of the dam grouping.